'If Britain is going to investigate journalists as terrorists, destroy documents, force us to give up passwords – then how can we be sure we can protect our sources?' Photograph: Kacper Pempel/Reuters

Free speech and freedom of the press are under attack in the UK. I cannot return to England, my country, because of my journalistic work with NSA whistleblower Edward Snowden and at WikiLeaks. There are things I feel I cannot even write. For instance, if I were to say that I hoped my work at WikiLeaks would change government behaviour, this journalistic work could be considered a crime under the UK Terrorism Act of 2000.

The act gives a definition of terrorism as an act or threat "designed to influence the government", that "is made for the purpose of advancing a political, religious, racial or ideological cause" and that would pose a "serious risk" to the health or safety of a section of the public. UK government officials have continually asserted that this risk is present with the disclosure of any "classified" document.

Elsewhere the act says "the government" means the government of any country – including the US. Britain has used this act to open a terrorism investigation relating to Snowden and the journalists who worked with him, and as a pretext to enter the Guardian's offices and demand the destruction of their Snowden-related hard drives. Britain is turning into a country that can't tell its terrorists from its journalists.

The recent judgment in the Miranda case proves this. David Miranda was assisting journalist Glenn Greenwald and transited through Heathrow with journalists' documents when he was held under schedule 7 of the Terrorism Act last summer. Schedule 7 means a person can be stopped and detained at a UK port for up to nine hours and affords no right to silence. It compels you to answer questions and give up any documents you possess, and so forced Miranda to hand over his Snowden documents. Subsequently Miranda fought a case against the UK government over the legality of his detainment, to show how this act infringes upon journalists' ability to work freely. Outrageously, the court found politically transparent excuses to ignore the well-defined protections for freedom of expression in the European convention on human rights.

If Britain is going to investigate journalists as terrorists take and destroy our documents, force us to give up passwords and answer questions – how can we be sure we can protect our sources? But this precedent is now set; no journalist can be certain that if they leave, enter or transit through the UK this will not happen to them. My lawyers advise me not to return home.

Snowden's US legal adviser, Jesselyn Radack, was questioned about Julian Assange and her client when she entered the UK recently. I am strongly connected to both men: I work for one, and rescued and watched over the other for four months. In addition, if schedule 7 is used to stop me upon entering the country . I could not answer such questions or relinquish anything, as this would be a risk to WikiLeaks's journalistic work, our people and our sources. As I would have no right to silence under this act, I would be committing a crime in the government's eyes. A conviction for "terrorism" would have severe consequences for free movement across international borders.

Schedule 7 is not really about catching terrorists, even in its own terms. The Miranda judgment states that it has in this case "constituted an indirect interference with press freedom" and is admittedly "capable, depending on the facts, of being deployed so as to interfere with journalistic freedom". Officers can detain someone not because they suspect them of being involved in terrorist activities, but to see "if someone appears" to – even indirectly – be facilitating the bizarre definition of terrorism used in the act.

Mr Justice Ouseley, who also presided over Assange's extradition case, stated in his judgment that an officer can act on "no more than hunch or intuition". It is now decreed by our courts that it is acceptable to interfere with the freedom of the press, based on a hunch – all in the name of "national security". Today instead of meaning "to ensure the stability of a nation for its people", national security is a catchphrase rolled out by governments to justify their own illegalities, whether that be invading another country or spying on their own citizens. This act – it is now crystal clear – is being consciously and strategically deployed to threaten journalists. It has become a tool for securing the darkness behind which our government can construct a brand new, 21st-century Big Brother.

This erosion of basic human civil rights is a slippery slope. If the government can get away with spying on us – not just in collusion with, but at the behest of, the US – then what checks and balances are left for us to fall back on? Few of our representatives are doing anything to act against this abusive restriction on our press freedoms. Green MP Caroline Lucas tabled an early day motion on 29 January but only 18 MPs have signed it so far.

From my refuge in Berlin, this reeks of adopting Germany's past, rather than its future. I have thought about the extent to which British history would have been the poorer had the governments of the day had such an abusive instrument at their disposal. What would have happened to all the public campaigns carried out in an attempt to "influence the government"? I can see the suffragettes fighting for their right to vote being threatened into inaction, Jarrow marchers being labelled terrorists, and Dickens being locked up in Newgate prison.

In their willingness to ride roughshod over our traditions, British authorities and state agencies are gripped by an extremism that is every bit as dangerous to British public life as is the (real or imaginary) threat of terrorism. As Ouseley states, journalism in the UK does not possess a "constitutional status". But there can be no doubt that this country needs a freedom of speech roadmap for the years ahead. The British people should fight to show the government we will preserve our rights and our freedoms, whatever coercive measures and threats it throws at us.